Human vision starts when photoreceptors collect and respond to light. Normal photoreceptor function and its support from the underlying retinal pigment epithelium (RPE) are essential for normal vision, yet techniques to assess these processes in vivo are limited. Current optical and electrophysiological techniques have limited spatial resolution and sensitivity, and target only specific functional processes. New optical modalities that are rapid, specific, and noninvasive hold the promise of greatly expanding our capability to monitor more accurately and completely the photoreceptor RPE complex. We will use adaptive optics (AO) and optical coherence tomography (OCT) to achieve unprecedented 3D resolution for studying physiological mechanisms at the cellular level in the photoreceptor and RPE layers. Further improvements will be realized using AOOCT in conjunction with an exquisitely sensitive phase technique that we have developed and that measures optical length changes as small as 45nm.This is slightly thicker than a single cone outer segment (OS) disc and nearly 100times better than the axial resolution of ultra high resolution OCT. The specific aims are to: (1) Determine the physical parameters that control light capture in photoreceptors; (2) Evaluate properties of light scatter in photoreceptors; and (3) Evaluate properties of light scatter in RPE.